Solar-powered plane’s transcontinental flight

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MOFFETT FEDERAL AIRFIELD, Calif. — The first plane that can fly day and night powered only by the sun began a transcontinental journey that will reach Washington by mid-June.

Solar Impulse, which lifted off from this World War II-era airfield on Friday, has room for only one person and an average cruising speed of about 43 miles per hour. But its Swiss developers say the technology suggests the possibilities of clean-energy flight.

The plane has an ultralight, carbon-fiber frame that allows it to weigh 3,500 pounds — about the same as a midsize car. It has the wingspan of a 747 and a slender fuselage, giving it the look of a giant, high-tech dragonfly.

The plane’s power is drawn from the sun by 12,000 photovoltaic cells that form the top of its wings. The juice is collected in a series of batteries arrayed behind the craft’s four electric engines. It routinely reaches altitudes of up to 28,000 feet, about a mile below the thin air traversed by big commercial airliners zipping around at close to 500 miles per hour.

For all of its innovations, at this stage of development, Solar Impulse is no more practical for commercial flight than was the single-engine Spirit of St. Louis that Charles Lindbergh piloted across the Atlantic Ocean in 1927.

The tiny cockpit is unheated and unpressurized, meaning the pilot must endure extreme heat and cold and wear an oxygen mask. On long flights, Borschberg practices meditation and advanced breathing techniques to stay energized. His co-founder and the plane’s other pilot, Bertrand Piccard, a psychiatrist, does self-hypnosis.

But comfort is not the project’s goal. ‘‘The point of this is to underscore how far we’ve come and how far we need to go to develop alternative sources of power, particularly solar energy,’’ said Bob van der Linden, chairman of the aeronautics department at the Smithsonian National Air and Space Museum. ‘‘This will help push the technology along.’’

He said there are some possible applications raised by Solar Impulse’s innovations, including high- and long-flying unmanned planes that could be used for mapping purposes. But any broad commercial uses, he said, lie beyond the horizon.

None of this dims the enthusiasm of the project’s founders.

In all, the project has cost more than $140 million. To date, the plane has performed well.

Borschberg piloted the first solar-powered night flight in 2010, spending 26 consecutive hours over Switzerland. In 2011, it flew from Switzerland to Belgium and then on to France, where it was a showpiece at the Paris-Le Bourget International Air Show. And last summer, the plane crossed the Mediterranean from Spain to Morocco.

After being transported here aboard a 747 cargo plane, Solar Impulse has soared over the Golden Gate Bridge, raising curiosity across the Bay Area and drawing several thousand visitors to its temporary home inside a cavernous wooden hangar that a couple of generations ago housed military dirigibles.

The plane will make five stops on its flight across the United States, spending up to a couple of weeks in each metropolitan area it visits: Phoenix, Dallas, St. Louis, and Washington, where it is slated to land at Dulles International Airport in Northern Virginia. From there, it is on to New York.